Gas turbine aircraft engines are typically designed to operate most efficiently at one particular condition which comprises a relatively narrow range of rotational speed for the turbine and compressor combination. For single spool applications where the turbine and compressor are mounted on a common shaft and rotate at the same speed, the usefulness in terms of driving a propulsion load such as a propeller, a fan or propfan may be limited whether the propulsion load is driven directly or via a reduction gearbox. Since the operational speed range of the single spool turbine engine is relatively limited, the single spool turbine engine is not particularly suitable for varying the power delivered to the propulsion load which is normally required for smaller aircraft applications. Therefore, the single spool turbine engine has limited applications due to the restricted range of power variability available to the operator.
Accordingly, there is a need to provide a gas turbine aircraft engine with increased power variability.